Exhaust gas emitted from an internal combustion system such as an automobile engine comprises toxic components such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM). Conventionally, to efficiently eliminate these exhaust components, an exhaust purification catalyst having a catalytic layer is used, with the catalytic layer comprising a carrier and a catalytic metal supported on the carrier.
For instance, a wall-flow-type exhaust gas purification catalyst comprises a wall-flow-type substrate and a catalytic layer. The wall-flow-type substrate has an entrance cell that is open on the exhaust inlet-side end, an exit cell that is open on the exhaust outlet-side end, and a porous partition wall (a ribbed wall) separating the two cells. The catalytic layer is provided to the interior or to the surface of the partition wall. Exhaust gas emitted from the internal combustion system flows via the exhaust inlet-side end into the entrance cell, passes through micro pores of the porous partition wall, and flows out of the exhaust outlet-side end of the exit cell. During this, upon contact between the exhaust gas and the catalytic layer (catalytic metal), the toxic components are purified (detoxified).
As for the art related to this, Patent Documents 1 to 4 are cited. For instance, Patent Document 1 discloses an exhaust gas purification catalyst comprising a catalytic layer having a two-layer structure. In particular, it discloses an exhaust gas purification catalyst that has a first catalytic layer (a Pd-containing layer) in the interior of the partition wall and a second catalytic layer (a Rh-containing layer) over the entire surface of the partition wall to completely cover the first catalytic layer, wherein solely the second layer additionally comprises a ceria-zirconia composite oxide capable of absorbing oxygen.